1. Field of the Invention
The invention relates to an installation and a process for the reconstruction of three-dimensional images.
2. Description of the Prior Art
The objects whereof it is wished to reconstruct the image by a tomography process, particularly in medicine or non-destructive testing in industry, are placed in front of a two-dimensional array of sensors rotating about them. The array is covered with a collimator, i.e. a plate made from lead or a similar material, which limits the space scanned by each detector to a single line. The array is passed over a circle or another closed trajectory about the object or a helical trajectory, and the collation of the measurements of the detectors, collected for several bidimensional images taken with the same number of different incidences, makes it possible to reconstruct a three-dimensional image of the object by using an analytical method for inverting a mathematical transform of the measurements or an algebraic resolution for inverting a system of equations. The function of the object or the property characterizing the image and which is measured by the detectors is either the emission of a radiation by the points of the object, or the attenuation by the points of the object of a radiation emitted by a source beyond the object on the focusing lines.
The location of the focusing points of the detectors, i.e. intersection points of the focusing lines, can assume different forms. It has thus been proposed to use a purely conical collimation, where there is a single focusing point for each two-dimensional image, which makes it possible to only use a point source. Earlier works carried out by one of the inventors and by other scientists have proved the value of this solution when using the inversion of what is called the primary of highest derivative of the Radon transform of the function in order to reconstitute the images, particularly in European patent 292 402, or more generally the sums of the function on planes.
It has also been proposed to focus the detectors on a straight line parallel to the rotation axis of the array, i.e. the focusing lines are arranged in fan form on staged planes and parallel to the trajectory. This "parallel fan" focusing mode, described in a certain number of articles, is disadvantageous due to the absence of magnification in the rotation axis.
More complex focusing locations are proposed in U.S. Pat Nos. 4,820,924 and 4,823,017. The collimating lines of the horizontal and vertical rows (rows and columns) of detectors converge in at least certain cases on the lines perpendicular to the array. In one embodiment, the focusing location is a straight line parallel to the array and perpendicular to the rotation axis. However, then the reconstruction calculation become very complicated.
The article by Muller, Arce and Blake entitled "Truncation artifacts and synthetic scanner arrays in two-dimensional computerized tomography", published in the reports of ECAPT, Karlsruhe, 1993 describes a method for the reconstruction of three-dimensional images, which comprises a stage of converting the projections into virtual projections in order to reduce the effects of the truncation of the measurements. The array of detectors makes it possible to acquire a plurality of truncated projections in conical geometry, as a result of the circular movement of the detector array and which is independent of that of the source and the virtual projections take place on a planar, virtual array of sensors. The invention also involves a conversion of real projections into virtual projections, but under different acquisition conditions, because the focusing is conical or in parallel fans in said article.